[smuxi/stable] Frontend-GNOME: fix NRE thrown in Frontend.Quit()

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit 51d0f49e205694a84754bb9353bb2ecf61da9a4d
Author: Mirco Bauer <meebey meebey net>
Date:   Mon Jan 25 18:05:05 2016 +0100

    Frontend-GNOME: fix NRE thrown in Frontend.Quit()
    
    If starting Smuxi fails and thus _MainWindow hasn't been initialized yet then
    calling Quit() throws a NullReferenceException.
    
        Unhandled Exception:
        System.NullReferenceException: Object reference not set to an instance of an object
          at Smuxi.Frontend.Gnome.Frontend.Quit () [0x00011] in 
/home/meebey/Projects/smuxi-stable/src/Frontend-GNOME/Frontend.cs:468
          at Smuxi.Frontend.Gnome.Frontend.ShowException (Gtk.Window parent, System.Exception ex) [0x00149] 
in /home/meebey/Projects/smuxi-stable/src/Frontend-GNOME/Frontend.cs:640
          at Smuxi.Frontend.Gnome.Frontend.ShowException (System.Exception ex) [0x00023] in 
/home/meebey/Projects/smuxi-stable/src/Frontend-GNOME/Frontend.cs:647
          at Smuxi.Frontend.Gnome.MainClass.Main (System.String[] args) [0x0026d] in 
/home/meebey/Projects/smuxi-stable/src/Frontend-GNOME/Main.cs:145

 src/Frontend-GNOME/Frontend.cs |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/Frontend-GNOME/Frontend.cs b/src/Frontend-GNOME/Frontend.cs
index cf56b0a..805d43d 100644
--- a/src/Frontend-GNOME/Frontend.cs
+++ b/src/Frontend-GNOME/Frontend.cs
@@ -465,7 +465,7 @@ namespace Smuxi.Frontend.Gnome
             Trace.Call();
 
             // only save windows size when we are not in the engine manager dialog
-            if (_MainWindow.Visible) {
+            if (_MainWindow != null && _MainWindow.Visible) {
                 // save window size
                 int width, heigth;
                 if (_MainWindow.IsMaximized) {